1. Technical Field
This invention relates generally to the manufacture of integrated circuit (IC) chips, and more specifically, to a photoresist material that includes both positive tone and negative tone attributes.
2. Background Art
Manufacturing of semiconductor devices is dependent upon the accurate replication of computer aided design (CAD) generated patterns onto the surface of a device substrate. The replication process is typically performed using lithographic processes followed by a variety of subtractive (etch) and additive (deposition) processes.
Photolithography, a type of lithographic process, is used in the manufacturing of semiconductor devices, integrated optics, and photomasks. The process basically comprises: applying a layer of a material that will react when exposed to light, known as a photoresist or, simply, a resist; selectively exposing portions of the photoresist to light or other ionizing radiation, i.e., ultraviolet, electron beams, X-rays, etc., thereby changing the solubility of portions of the material; and developing the resist by washing it with a basic developer solution, such as tetramethylammonium hydroxide (TMAH), thereby removing the non-irradiated (in a negative resist) or irradiated (in a positive resist) portions of the layer.
Conventional positive and negative tone photoresists are characterized by a dissolution curve in which there is a single transition from a first dissolution rate to a second dissolution rate as the resist is exposed to varying levels of actinic radiation. In a positive resist, the initially unexposed resist is insoluble in developer, while the exposed resist becomes more soluble as the exposure dose is increased above a threshold value. For a negative resist, similar behavior is observed, except that the initially unexposed resist is soluble in developer, and the exposed area is rendered insoluble. By means of this differential solubility between the exposed and unexposed resist areas, it is possible to form a pattern in the resist film. This pattern can be used to form integrated circuit devices, and is currently a critical component in their manufacture.
In an ideal situation, the exposure tool would only allow the radiation to hit the resist material in the areas of the mask that are clear, thus providing sharp edges for the lines and spaces. However, diffraction patterns are formed at the edges of the clear areas, resulting in partial exposure of the resist in those areas. Certain patents have taken advantage of this phenomenon, such as U.S. Pat. No. 4,568,631 issued to Badami et al. on Feb. 4, 1986 and assigned to the assignee of record for the present invention, which discloses utilizing a positive resist and an additive for image reversal in order to create thin resist lines only in the areas where light intensity has been reduced by diffraction effects. However, this procedure uses a resist with conventional positive and negative tone dissolution responses and requires two separate expose and develop operations to form a resist image from the edge of a reticle image.
As the need for higher and higher levels of integration has arisen in the industry, the need for a larger number of lines and spaces in a given area has increased dramatically. In response, a primary subject of research has been enhancement of the exposure tool and reticle system to enhance the aerial image of the circuit pattern. For example, phase shift reticles, shorter wavelength expose tools, higher numerical aperture expose tools, and tools with selective illumination systems are continuing to be developed to improve the pattern density of integrated circuits. Due to the high cost, poor yield, and difficulty of inspection and repair, phase shift reticles are generally not available for use. Due to the complexity of exposure tool design and construction, it is very expensive to build higher numerical aperture and shorter wavelength expose systems.
In another area of activity, efforts are being made to improve the contrast of the photoresist. However, the basic mechanism of operation of the photoresist continues to be the same; that is, the resists behave as either positive or negative tone systems. It is desirable, therefore, to devise new a mechanisms of resist response such that conventional optical lithography can be extended to smaller feature sizes without developing new tools and reticles. Additionally, as these new tools and reticles are eventually developed and implemented, these new resist approaches would remain applicable as a further extension of lithographic capability.
Presently, for high performance devices, the control of the image size on the reticle and the control of image size from one batch of wafers to the next comprise the largest contributors to image size variation on the product. Chip yield at high performance is strongly dependent on the uniformity of the image pattern across the chip and the centering of the image pattern at the correct dimension. These limitations exist currently for all types of lithographic patterning which use a reticle; optical, x-ray, and proximity E-beam, for example The problem of image uniformity across the reticle is especially acute for lithographic techniques that use 1xc3x97 masks, such as x-ray and proximity E-beam lithography. It is therefore desired to provide a photoresist material that allows very precise image control for the image size, independent of the image size control on the reticle.
Accordingly, the present invention provides a photoresist material having, simultaneously, both a positive tone and a negative tone response to exposure. This combination of materials can provide a new type of resist, which we call a hybrid resist.
As a hybrid resist is exposed with actinic radiation, areas exposed with high intensity radiation form a negative tone line image. Areas which are unexposed remain insoluble in developer, thus forming a positive tone line pattern. Areas which are exposed with intermediate amounts of intensity, such as the edges of the aerial image where diffraction effects have reduced the intensity, form a space in the resist film during develop. This resist response is an expression of the unique dissolution rate properties of this resist, in which unexposed resist does not develop, partially exposed resist develops at a high rate, and highly exposed resist does not develop.
It is, therefore, a feature of the present invention that the unique dissolution rate response of the hybrid photoresist allows a single aerial image to be printed as a space/line/space combination rather than as a single line or space, as with conventional resist. This xe2x80x98frequency doublingxe2x80x99 capability of this resist allows conventional expose systems to be extended to higher pattern densities. It is an advantage of one example of the present invention that lines and spaces of 0.2 xcexcm and less can be printed with current deep ultra violet (DUV) lithography tools that are designed for operation at 0.35 xcexcm resolution.
It is a further advantage of this type of hybrid resist that the space width is generally unchanging as the exposure dose and the reticle image size are changed. This allows very precise image control for the space width within each chip, across each wafer, and from one batch of product wafers to the next.
Still another advantage of this invention is the relaxation of the minimum reticle feature size due to the frequency doubling capability of hybrid resist. For example, to print a 0.2 xcexcm feature with conventional resist generally requires a 0.2 xcexcm reticle image size. With hybrid resist, a 0.2 xcexcm space can be formed with a single edge of a reticle feature; for example, a 0.5 xcexcm reticle opening could produce two 0.2 xcexcm spaces and a 0.2 xcexcm line. In this way, one could accomplish xe2x80x98reductionxe2x80x99 x-ray or E-beam lithography; the reticle image pitch could be approximately 2xc3x97 the printed pitch on the substrate. This also has the additional advantage of allowing a relaxation of the image size requirements of optical reticles, reducing cost and improving yield of the reticle.
It is an advantage of the present invention that lines and spaces of 0.2 xcexcm and less may be achieved without altering the present tools.
It is a further advantage that the space width is generally unchanging as the exposure dose and reticle sizes change, thereby allowing greater process latitude for control of space width. Through the use of the hybrid resist of the present invention, errors in the image dimension on the reticle are not reproduced in the space width printed on the substrate. As a result, the across-chip space width variation is minimized. This is valuable for optical, X-ray and e-beam exposure methods. It is especially useful in lithographic techniques that require a 1xc3x97 reticle, i.e., a reticle that normally has a one-to-one relationship with the image printed on the substrate, because variations in the image size on the reticle are normally reproduced on the substrate.
The foregoing and other features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention.